1. Field of the Invention
The present invention relates to a process for producing a structural member of aluminum alloy, and particularly, to a process for producing a structural member of aluminum alloy by subjecting a powder preform of aluminum alloy powder to a heating treatment and then to a compacting and hardening process under a pressure.
2. Description of the Prior Art
There is such a conventionally known process for producing a structural member having a fine metallographic structure using an aluminum alloy powder having a non-equilibrium phase (for example, see Japanese Patent Application Laid-open No. 279767/93).
In the heating treatment in the known process, the rapid increase in temperature of the powder preform is conducted at an average temperature-increasing rate R equal to or higher than 333 K./min from room temperature to a forging temperature.
The reason why such a rapid increase in temperature is conducted is that the thermal hysteresis of the powder preform is decreased and hydrogen is rapidly released from the powder preform, so that the powder preform is veiled in hydrogen and thus prevented from being oxidized.
However, when an aluminum alloy powder having a non-equilibrium phase showing a calorific value C equal to or higher than 10 J/g at a temperature-increasing rate of 20 K./min in a differential scanning calorimetry is used for the purpose of further refining the metallographic structure of the structural member, if the rapid increase in temperature equivalent to that in the known process is conducted, a problem arises that the phase change is not uniformly conducted in the powder preform and, as a result, the produced structural member has an non-uniform metallographic structure and hence, has lower mechanical characteristics.
To solve this problem, it is necessary to lower the average temperature-increasing rate during the phase change down to a value lower than that in the known process. On the other hand, it is necessary to rapidly generate the releasing of hydrogen after the phase change and hence, it is desirable to increase the average temperature-increasing rate to correspond to this.